1. Field of the Invention
The present invention relates to a converter and a conversion method of DC-to-DC step-up, and more particularly to a converter and a conversion method of DC-to-DC step-up, which is based on pulse frequency modulation (PFM). The present invention is especially suitable for applications of low voltage and low current.
2. Description of the Related Art
Recently the development of all consumer electric products such as personal digital assistants (PDAs), smart phones, the Walkman, and so on has focused on seeking a longer operating time. Batteries are developed to meet the requirements of endurance and portability. As we know, the capacity of batteries is limited, and accordingly how to utilize batteries is getting more important. When the required current of internal components in a system decreases, a PFM convert is likely to be the best solution for applications of low load and low current.
In general, there are two methods of implementing a step-up converter using the PFM technique. The first method is to use an output signal of an error amplifier to control the frequency of a voltage-controlled oscillator (VCO). The VCO is designed to be configured with fixed high-level period and adjustable low-level period. The disadvantage of this method is that the frequency of the error amplifier requires compensation. If a compensation circuit is combined in the error amplifier, the area of the error amplifier chip will increase. If the layout of the compensation circuit is outside the error amplifier, an additional pin would be provided to receive the compensation signal.
Referring to FIG. 1, the second method is to use an output signal of a comparator 11 to control an output square wave of a ring oscillator 12. After passing a buffer 13, the output square wave drives a step-up circuit 14. A DC output voltage Vout of the step-up circuit 14 is then sent through a feedback path 15 to generate a feedback signal FBO by divided resistors R10 and R20. The feedback signal FBO is further fed to the comparator 11. When the DC output voltage Vout is stable and then the external load suddenly drops, the energy stored in an inductor of the step-up circuit 14 is released to the external load. As a result, noises are induced. The noises are fed through the feedback path 15 and enter the comparator 11, and finally cause ripples i n the output signal of the ring oscillator 12, in which the output signal is originally a square wave. In addition, to obtain a linear operation of a PFM converter, the high-level period of the output square wave of the ring oscillator 12 has to be fixed. If the last square pulse of the output signal is outputted and then a DC output voltage impulse is caused by relief of excessive energy stored in the inductor of the step-up circuit 14, the last square pulse will be cut off (i.e., the high-level period is reduced) and will result in reduction of output energy. Consequently, the DC output voltage Vout drops, and ripples occur.